Influenza A viruses have been responsible for large losses of life around the world and continue to present a great public health challenge. These viruses can cause infections in birds, sea mammals, lower mammals (e.g., pigs, dogs, and horses), and ulfimately humans. Previous studies have demonstrated that the structures of the carbohydrate receptors determine influenza host and tissue tropisms. However, the distribution and composifion ofthe endogenous receptors are not well characterized, and the distribution of such structures within and across the host species is not fully understood. The goal of this proposal is to understand the receptor profiles for influenza A virus in its natural hosts and to identify any novel receptor specificities for influenza viral infection. We will use H1 Nl influenza A viruses as a model since HI Nl viruses caused 1918 and 2009 influenza pandemic as well as seasonal influenza since 1977 and H1N1 influenza A viruses were also detected in birds and swine . We hypothesize that, in addition to SA2,3Gal and SA2,6Gal, other glycan epitopes could contribute to H1N1 influenza host/tissue tropisms. We propose to characterize host receptors for HI Nl influenza A viruses in pigs and birds, including both waterfowl and land-based birds using omics and systems biology approaches. We will test this hypothesis by conducfing the following two specific aims: (1) Characterize the H1N1 influenza receptors in pigs and birds using immunostaining analysis and mass spectrometry technology; (2) Identify and validate the conserved/unique structural glycan moieties and potentially novel receptor determining H1N1 influenza host/tissue tropisms.